Mad (Flt) Scientist is absolutely right when he says the MCAS is unable to prevent the stall. I have no idea exactly how or when the MCAS operates – I bow to FCeng84’s knowledge of the system and accept his description. Equally, I was never a 737 pilot, but I do know a little about stalling large transport aircraft, hence my attempt to try to tease out the differences/dividing line between stall protection devices and the enhancement of handling qualities before, during and after the stall.
MCAS does not prevent a stall, what it is intended to do is it redress the degraded longitudinal stability prior to and at the stall. It is all about stability and handling qualities. I first came across these issues on the VC10. Since it was a T-tail aircraft with the potential to become locked into a deep stall, it was fitted with a stick pusher which pushed after the natural aerodynamic buffet started and after the stick pusher activated but before a pre-determined AoA – i.e. before the ‘natural stall’ itself. Thus it prevented any tendency to pitch up into a deep stall and defined the stall point for certification purposes. You could say it was the ultimate anti-stall device.
On the other hand, the 747's stall qualities were immaculate except in the clean configuration when the stick force reduced to zero shortly before the stall and it would have gently progressed all the way into the stall on its own. Because of this, the UK CAA (D.P.Davies – see first sticky in Tech Log) required a stick nudger to activate at the same time as the shaker. It gave a small push (only about 15lbs) but just enough to increase the stick force and thus restore the required longitudinal stability.
The point I was trying to make in my previous posts is where do improvements to the longitudinal stability end and where do stall detection and protection systems start. Are subtle changes to the wing profile anti-stall devices or are they there to achieve certifiable handling qualities pre-stall? Is some kind of stability augmentation device an anti-stall device or is it there to provide a small force into the elevator circuit to impose positive stick free stability and thus remove the otherwise self-stalling tendency?
It seems therefore as zzuf has stated, that the MCAS was a response to a longitudinal static stability deficiency approaching the stall. And Boeing decided to do so by increasing the stick force by moving the stabiliser.
So, I leave you, the reader, to decide whether MCAS is an anti-stall device or, as safetypee says, a ‘crutch’ to address more specific nose up issues when approaching or at the stall (25.203) and, when turning, where the trim would be more nose up, and where there may be a greater pitching moment associated with the new engines on the 737 MAX.